JPH09125194A - High strength hot rolled steel sheet excellent in stretch-flanging property and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide technology for producing a high strength hot rolled steel sheet having high strength of 60 to 100kgf/mm<2> and having excellent stretch- flanging properties in accordance with strength. SOLUTION: As for a steel slab having a compsn. contg., by weight, <=0.18% C, 0.5 to 2.5% Si, 0.5 to 2.5% Mn, <=0.05% P and <=0.02% S, furthermore contg. one or two kinds selected from <=0.5% Ti and <=1.0% Nb, in which the above C is regulated so as to satisfy the following inequality in the relation with Nb and Ti, and the balance Fe with inevitable impurities, hot rolling is finished at >=800 deg.C. Next, within 5sec, this rolled stock is cooled at a cooling rate of >=20 deg.C/sec, is retained in the temp. range of 650 to 750 deg.C for >=5sec, is subsequently cooled at a cooling rate of >=10 deg.C/sec and is coiled at <=540 deg.C to form a steel structure contg. ferrite having <=25μm grain size in which Ti and/or Nb carbides are precipitated and having 70 to 95% area ratio, and the balance martensite or martensite and residual austenite: the inequality; C>=0.03+Ti/4+ Nb/7.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to hot-rolled steel sheets, and particularly to high-strength hot-rolled steel sheets for processing which are suitable for use in press forming of automobile underbody parts and the like, and particularly have a tensile strength of 60 to 100 kgf /
The present invention relates to a high-strength hot-rolled steel sheet having a high tensile strength of mm ² and having excellent stretch-flangeability and good workability, and a method for producing the same.

[0002]

2. Description of the Related Art Steel sheets used for undercarriage parts and strength members for automobiles are conventionally high-strength steel sheets (hereinafter simply referred to as "high tensile strength") for the purpose of reducing vehicle weight.
Has been used. Such a high tensile strength steel for automobiles is required to have a certain level of strength in order to ensure safety, and at the same time, it is required to have good moldability represented by press working. In particular, in recent years, the strength level of wheels, bumper inners, bumper stays, etc. has come to be demanded of 80 kgf / mm ² class high tensile strength, and for the material supply side, it is the same as the conventional 45 to 55 kgf / mm ² class. The development of high-tensile high-tensile products that can be molded with the same equipment and the same number of processes is a major issue.

By the way, there are many options for strengthening conventional steel sheets up to the TS 60 kgf / mm ² class, such as solid solution strengthening, structure strengthening, precipitation strengthening, and grain refining strengthening. It was possible to make different models by taking advantage of their economic characteristics. However, when it comes to steel sheets of 60 kgf / mm ² grade or higher,
The options are considerably less. First, it is difficult to achieve the target strength by grain refinement strengthening or solid solution strengthening. Next, regarding precipitation strengthening, it was difficult to achieve strength only by precipitation strengthening, and in reality there was a hard second phase,
There is no choice but to rely on a complex mechanism with organizational strengthening. In view of such a current situation, as a method for achieving both strength and workability, in JP-A-5-179396,
A low-yield-ratio high-strength hot-rolled steel sheet and a method for producing the same are disclosed, which have both the characteristics of precipitation strengthening and microstructure strengthening.

[0004]

However, in the case of high tensile strength steel using such a hard second phase, the local deformability is lowered due to the expansion of the interphase strength difference between the soft phase and the hard phase,
Deterioration of stretch flangeability became remarkable, and it was the main subject in the past
When the same press working as 45 to 55 kgf / mm ² grade steel sheet was applied, there were problems such as cracking in the stretch flange portion. The method disclosed in JP-A-5-179396 is also excellent in that it achieves a low YR due to the remaining mobile dislocations introduced during the formation of the hard second phase, but it governs the stretch-flangeability described above. The homogeneity of the macroscopic structure, the grain size of the structure, etc. are not taken into consideration, and therefore there is a problem that a satisfactory stretch-flangeability according to the strength, so-called strength-stretch-flangeability, has not yet been obtained. Was there.

Therefore, an object of the present invention is to advantageously solve the above-mentioned problems that the above-mentioned conventional techniques have.
An object of the present invention is to provide a manufacturing technology of a high-strength hot-rolled steel sheet having a high strength of 60 to 100 kgf / mm ² grade and excellent stretch-flangeability depending on the strength. Another object of the present invention is to strengthen the soft ferrite phase by precipitation strengthening with Ti and / or Nb to reduce the interphase strength difference between the hard second phase and the ferrite grain size and the area ratio occupying the whole thereof. by controlling the cooling condition, by improving the homogeneity of the tissue was solved the aforementioned problems associated with high tension reduction, 60 to 100 [kgf / mm ² class high strength hot rolled with excellent stretch flangeability To provide steel plate manufacturing technology.

[0006]

[Means for Solving the Problems] As a result of conducting many experiments on the composition of components and manufacturing conditions, the inventors have found that the relationship between C and Ti and / or Nb is appropriate based on the conventional precipitation strengthened steel. By controlling the range and adjusting and adding Si, by appropriately controlling the cooling conditions after hot rolling, precipitation strengthening occurs at the same time as the γ → α transformation after hot rolling and is discharged from the ferrite grains. C is concentrated in untransformed γ grains and finally the precipitation-strengthened ferrite phase is the main component, and the second phase is a martensite phase or a composite structure containing a small amount of martensite phase and residual γ phase. By reducing the strength difference between the soft phase and the hard phase, and by homogenizing and refining the structure, the strength of TS60kgf / mm ^{2 to} 100kgf / mm ² is secured as a whole structure and stretch flangeability is achieved. Depending on strength Then, they found that they could be improved, and completed the present invention.

That is, the gist of the present invention is as follows. 1) C: 0.18 wt% or less, Si: 0.5 to 2.5 wt%, Mn: 0.
5 to 2.5 wt%, P: 0.05 wt% or less, S: 0.02 wt% or less, and any one or two selected from Ti: 0.5 wt% or less and Nb: 1.0 wt% or less The above C is adjusted so as to satisfy the following formula in relation to Nb and Ti, and has a composition in which the balance is Fe and inevitable impurities, and its structure is Ti and / or Nb.
A high-strength hot-rolled steel sheet having an area ratio of 70 to 95% of ferrite having a grain size of 25 μm or less, in which carbides are precipitated, and the balance being martensite or martensite and retained austenite and having excellent stretch flangeability. Formula C ≧ 0.03 + Ti / 4 + Nb / 7

2) C: 0.18 wt% or less, Si: 0.5 to 2.5
wt%, Mn: 0.5 to 2.5 wt%, P: 0.05 wt% or less, S:
0.02wt% or less, Al: 0.10wt% or less, and Ti:
0.5 wt% or less and Nb: 1.0 wt% or less, and the above C contains Nb,
Adjusted to satisfy the following formula in relation to Ti,
The balance has a composition consisting of Fe and inevitable impurities, and the structure is such that ferrite with Ti and / or Nb carbide having a grain size of 25 μm or less has an area ratio of 70 to 95%, and the balance is martensite. Alternatively, a high-strength hot-rolled steel sheet composed of martensite and retained austenite and having excellent stretch-flangeability. Formula C ≧ 0.03 + Ti / 4 + Nb / 7

3) C: 0.18 wt% or less, Si: 0.5 to 2.5
wt%, Mn: 0.5 to 2.5 wt%, P: 0.05 wt% or less, S:
0.02 wt% or less, Cr: 1.5 wt% or less, and T
i: 0.5 wt% or less and Nb: 1.0 wt% or less, and the above C contains
The composition is adjusted to satisfy the following formula in relation to Nb and Ti, and the balance has a composition consisting of Fe and unavoidable impurities, and its structure has a grain size of 25 μm or less in which carbides of Ti and / or Nb are precipitated. The area ratio of ferrite is 70 to 95
%, And the balance is martensite or martensite and retained austenite. Formula C ≧ 0.03 + Ti / 4 + Nb / 7

4) C: 0.18 wt% or less, Si: 0.5 to 2.5
wt%, Mn: 0.5 to 2.5 wt%, P: 0.05 wt% or less, S:
0.02 wt% or less, Al: 0.10 wt% or less, Cr: 1.5 wt% or less, and Ti: 0.5 wt% or less and
Nb: Containing one or two selected from 1.0 wt% or less, the above C is adjusted to satisfy the following formula in relation to Nb and Ti, and the balance is Fe and inevitable impurities. Elongation that has a composition and its structure is 70 to 95% by area ratio of ferrite with grain size of 25 μm or less where Ti and / or Nb carbides are precipitated, and balance is martensite or martensite and retained austenite High strength hot rolled steel sheet with excellent flangeability. Formula C ≧ 0.03 + Ti / 4 + Nb / 7

5) C: 0.18 wt% or less, Si: 0.5 to 2.5
wt%, Mn: 0.5 to 2.5 wt%, P: 0.05 wt% or less, S:
Contains 0.02 wt% or less and Ti: 0.5 wt% or less and Nb:
In addition to containing one or two selected from 1.0 wt% or less, the above C is adjusted so as to satisfy the following formula in relation to Nb and Ti, and the balance is composed of Fe and inevitable impurities. Hot rolling of steel slabs is completed at 800 ℃ or more, then cooled at a cooling rate of 20 ℃ / sec or more within 5 seconds, and 5 s in a temperature range of 750 ℃ or less and 650 ℃ or more.
A method for producing a high-strength hot-rolled steel sheet having excellent stretch-flange formability, which is characterized in that it is retained for c or more, then cooled at a cooling rate of 10 ° C / sec or more, and wound at a temperature of 540 ° C or less. Formula C ≧ 0.03 + Ti / 4 + Nb / 7

6) C: 0.18 wt% or less, Si: 0.5 to 2.5
wt%, Mn: 0.5 to 2.5 wt%, P: 0.05 wt% or less, S:
0.02wt% or less, Al: 0.10wt% or less, and Ti:
0.5 wt% or less and Nb: 1.0 wt% or less, and the above C contains Nb,
Adjusted to satisfy the following formula in relation to Ti,
A steel slab whose composition consists of Fe and inevitable impurities is hot-rolled at 800 ℃ or more, then cooled at a cooling rate of 20 ℃ / sec or more within 5 seconds, and cooled to 750 ℃ or less, 65
Let it stay in the temperature range of 0 ℃ or more for 5 seconds or more, then 10 ℃
A method for producing a high-strength hot-rolled steel sheet having excellent stretch-flange formability, which comprises cooling at a cooling rate of not less than / sec and winding at a temperature of 540 ° C or less. Formula C ≧ 0.03 + Ti / 4 + Nb / 7

7) C: 0.18 wt% or less, Si: 0.5 to 2.5
wt%, Mn: 0.5 to 2.5 wt%, P: 0.05 wt% or less, S:
Contains 0.02 wt% or less, Cr: 1.5 wt% or less, and Ti:
0.5 wt% or less and Nb: 1.0 wt% or less, and the above C contains Nb,
Adjusted to satisfy the following formula in relation to Ti,
A steel slab whose composition consists of Fe and inevitable impurities is hot-rolled at 800 ℃ or more, then cooled at a cooling rate of 20 ℃ / sec or more within 5 seconds, and cooled to 750 ℃ or less, 65
Let it stay in the temperature range of 0 ℃ or more for 5 seconds or more, then 10 ℃
A method for producing a high-strength hot-rolled steel sheet having excellent stretch-flange formability, which comprises cooling at a cooling rate of not less than / sec and winding at a temperature of 540 ° C or less. Formula C ≧ 0.03 + Ti / 4 + Nb / 7

8) C: 0.18 wt% or less, Si: 0.5 to 2.5
wt%, Mn: 0.5 to 2.5 wt%, P: 0.05 wt% or less, S:
0.02 wt% or less, Al: 0.10 wt% or less, Cr: 1.5 wt% or less, and Ti: 0.5 wt% or less and
Nb: Containing one or two selected from 1.0 wt% or less, the above C is adjusted to satisfy the following formula in relation to Nb and Ti, and the balance is Fe and inevitable impurities. The steel slab having the composition is hot-rolled at 800 ° C or higher, then cooled at a cooling rate of 20 ° C / sec or higher within 5 seconds, and allowed to stay in the temperature range of 750 ° C or lower and 650 ° C or higher for 5 seconds or longer, and thereafter. A method for producing a high-strength hot-rolled steel sheet having excellent stretch-flange formability, which comprises cooling at a cooling rate of 10 ° C / sec or more and winding at a temperature of 540 ° C or less. Formula C ≧ 0.03 + Ti / 4 + Nb / 7

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The component composition and manufacturing conditions will be described below. C: 0.18 wt% or less, C ≧ 0.03 + Ti / 4 + Nb / 7 (wt%) If C is contained in excess of 0.18 wt%, workability and spot weldability are severely deteriorated, so the upper limit is restricted. However,
In relation to Ti and Nb, C ≧ 0.03 + Ti / 4 + Nb /
When the condition of 7 (wt%) is not satisfied, this C is preferentially consumed in the precipitation reaction of TiC and NbC during the γ → α transformation, and the amount of C enriched in the untransformed γ grains becomes insufficient. It becomes impossible to achieve the strength. Therefore, the C content must satisfy the above formula and be contained in the range of 0.18 wt% or less.

Si: 0.5-2.5 wt% Si has the effect of promoting the precipitation of carbides in ferrite,
It is important because it has both the effect of martensite or the effect of making residual γ in the second phase, and the effect is exhibited by adding 0.5 wt% or more. However, when the amount exceeds 2.5 wt%, the effect is saturated, and the descaling property deteriorates and the cost increases, so the range is 0.5 to 2.5 wt%. The preferable addition range is 0.7 to 1.70 wt%.

Mn: 0.5-2.5 wt% Mn cannot obtain a desired composite structure when the content is less than 0.5 wt%, while excessively lowering the Ar ₃ transformation point when it exceeds 2.5 wt%. 0.5 to 2.5 wt% because α-grains are less likely to precipitate during cooling after rolling, which is an obstacle to precipitation strengthening.
Limited to. The preferable addition range is 1.2-1.8 wt%
It is.

P: 0.05 wt% or less P reduces the workability and weldability, so P is set to 0.05 wt% or less.

S: 0.02 wt% or less S reacts with Mn in steel to form A-type inclusions and deteriorates stretch flangeability, so S is limited to 0.02 wt% or less.

Ti: 0.5 wt% or less, Nb: 1.0 wt% or less Ti and Nb are elements that play an important role in the present invention, and carbides in α grains are simultaneously advanced during the γ → α transformation after rolling. And contributes to strengthening the base. But Nb, Ti
When the addition amount of is small, the precipitated particles become coarse and lose the precipitation strengthening ability, and the ratio of the second phase increases, and the stretch flangeability deteriorates. On the other hand, if the amount of addition is large, the amount of C necessary for forming the second phase is insufficient, and the characteristics tend to be precipitation strengthened, resulting in deterioration of the balance of processing characteristics and deterioration of ductility. For this reason, Ti is added in an amount of 0.5 wt% or less, preferably 0.02 to 0.5 wt%, and Nb is added in an amount of 1.0 wt% or less, preferably 0.02 to 1.0 wt%. Further, these Ti and Nb can be selectively used because they have a common effect, and one kind or two kinds are used within the above range.

Al: 0.10 wt% or less Al is an element useful for cleaning steel, but 0.10 wt%
If it exceeds%, it will cause clusters, etc., so 0.10wt%
Add in the following range. In the present invention, Si
It is also possible to carry out deoxidation only with Si added with 0.5 wt% or more.

Cr: 1.5 wt% or less Since Cr has an effect as a substitute for Mn, 1.5 wt%
It can be appropriately added within the following range.

Next, the manufacturing conditions will be described. First,
In hot rolling, the rolling end temperature shall be 800 ° C or higher. If this temperature is lower than 800 ° C, the ductility of the hot-rolled steel sheet is significantly deteriorated. In the hot rolling of the present invention, after the continuous casting slab is once cooled, it is reheated to perform rough rolling, or immediately after continuous casting or without heat treatment without lowering the temperature to 800 ° C or lower. It may be a case where rolling is performed later.

After completion of the above rolling, cooling is started within 5 seconds, and cooling is performed at a cooling rate of 20 ° C./sec or more to 750 ° C. or less.
It is necessary to stay in the temperature range of 50 ° C or less and 650 ° C or more for 5 seconds or more. After the rolling is completed, the elapsed time until the start of cooling and the cooling rate are particularly important constituent factors. If the elapsed time until the start of cooling exceeds 5 seconds or if the cooling rate is below 20 ° C / sec, The structure after rolling becomes coarse γ grains due to excessive recovery and recrystallization, and the subsequent α grains also become coarse, deteriorating the workability. Also, 750
If the residence time is less than 5 seconds at temperatures below 650 ° C and above 650 ° C, the structure is uniform hard bainite or bainitic ferrite + martensite due to insufficient concentration of C in γ grains due to insufficient precipitation of α grains and subsequent rapid cooling. (Partially retained γ), resulting in deterioration of workability such as ductility and stretch flangeability.

Next to the residence in a predetermined temperature range, 10 ° C./s
It is necessary to cool above ec and to wind up below 540 ° C. This is because if the cooling rate is less than 10 ° C / sec, pearlite will precipitate and the desired strength will not be obtained, and if the coiling temperature exceeds 540 ° C, the appearance amount of martensite will be insufficient and similarly. This is because it becomes difficult to obtain the desired strength. In this steel type, the area ratio of the hard second phase in the structure can be adjusted by adjusting the winding temperature, and the lower limit of the winding temperature is not particularly specified.

[0026]

EXAMPLE A steel slab adjusted to the composition shown in Table 1
Hot rolling was performed under various conditions to produce hot rolled steel sheets with a plate thickness of 3.2 mm. The tensile properties and stretch flangeability of the obtained hot rolled steel sheets were investigated. The tensile test was carried out in the L direction using a JIS No. 5 test piece. The hole expansion rate, which indicates stretch flangeability, was measured by using a 150 mmφ test piece with a punched hole of 36 mmφ (D _O ), pushing up the center of this test piece with a ball-head punch with a radius of 50 mm, and then microcracking. The diameter (D _I ) at which the occurrence of the phenomenon occurred was measured and calculated by the following formula.

(Equation 1)

[0027]

[Table 1]

Table 2 shows the hot rolling conditions and the test results. The metallic structure of the material of the present invention was composed of ferrite, martensite, and retained austenite. As is clear from Table 2, each of the invention examples has a tensile strength of 60 kgf / mm ² or more and exhibits a good hole expansion rate. That is, as shown in FIG. 1, it can be seen that the balance between strength and stretch-flangeability is significantly superior to that of the comparative example. Incidentally, the data of JP-A-5-179396 cited in the description of the prior art is also plotted in the same figure, and from this, the balance of strength and stretch flangeability of the invention example is far superior to that of the prior art. It can be seen that it has characteristics.

[0029]

[Table 2]

[0030]

As described above, according to the present invention, the tensile strength is controlled by using the conventional precipitation strengthening and the structure strengthening in combination, and by controlling the grain size of the precipitation strengthened ferrite to a fine grain. It is possible to manufacture a hot-rolled steel sheet having high strength of 60 kgf / mm ² or more and excellent stretch flangeability, and it is extremely useful as a hot-rolled steel sheet for press forming such as automobile underbody parts and strength members.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between tensile strength and hole expansion rate.

Claims (5)

[Claims] 1. C: 0.18 wt% or less, Si: 0.5 to 2.5 wt%, Mn: 0.5 to 2.5 wt%, P: 0.05 wt% or less, S: 0.02 wt% or less, and Ti: 0.5 wt% Below and
Nb: Containing one or two selected from 1.0 wt% or less, the above C is adjusted to satisfy the following formula in relation to Nb and Ti, and the balance is Fe and inevitable impurities. Elongation that has a composition and its structure is 70 to 95% by area ratio of ferrite with grain size of 25 μm or less where Ti and / or Nb carbides are precipitated, and balance is martensite or martensite and retained austenite High strength hot rolled steel sheet with excellent flangeability. Formula C ≧ 0.03 + Ti / 4 + Nb / 7 2. C: 0.18 wt% or less, Si: 0.5 to 2.5 wt%, Mn: 0.5 to 2.5 wt%, P: 0.05 wt% or less, S: 0.02 wt% or less, Al: 0.10 wt% or less And Ti: 0.5 wt% or less and
Nb: Containing one or two selected from 1.0 wt% or less, the above C is adjusted to satisfy the following formula in relation to Nb and Ti, and the balance is Fe and inevitable impurities. Elongation that has a composition and its structure is 70 to 95% by area ratio of ferrite with grain size of 25 μm or less where Ti and / or Nb carbides are precipitated, and balance is martensite or martensite and retained austenite High strength hot rolled steel sheet with excellent flangeability. Formula C ≧ 0.03 + Ti / 4 + Nb / 7 3. C: 0.18 wt% or less, Si: 0.5 to 2.5 wt%, Mn: 0.5 to 2.5 wt%, P: 0.05 wt% or less, S: 0.02 wt% or less, Cr: 1.5 wt% or less And Ti: 0.5 wt% or less and
Nb: Containing one or two selected from 1.0 wt% or less, the above C is adjusted to satisfy the following formula in relation to Nb and Ti, and the balance is Fe and inevitable impurities. Elongation that has a composition and its structure is 70 to 95% by area ratio of ferrite with grain size of 25 μm or less where Ti and / or Nb carbides are precipitated, and balance is martensite or martensite and retained austenite High strength hot rolled steel sheet with excellent flangeability. Formula C ≧ 0.03 + Ti / 4 + Nb / 7 4. C: 0.18 wt% or less, Si: 0.5 to 2.5 wt%, Mn: 0.5 to 2.5 wt%, P: 0.05 wt% or less, S: 0.02 wt% or less, Al: 0.10 wt% or less Cr: Includes less than 1.5 wt% and Ti: less than 0.5 wt% and
Nb: Containing one or two selected from 1.0 wt% or less, the above C is adjusted to satisfy the following formula in relation to Nb and Ti, and the balance is Fe and inevitable impurities. Elongation that has a composition and its structure is 70 to 95% by area ratio of ferrite with grain size of 25 μm or less where Ti and / or Nb carbides are precipitated, and balance is martensite or martensite and retained austenite High strength hot rolled steel sheet with excellent flangeability. Formula C ≧ 0.03 + Ti / 4 + Nb / 7 5. C: 0.18 wt% or less, Si: 0.5 to 2.5 wt%, Mn: 0.5 to 2.5 wt%, P: 0.05 wt% or less, S: 0.02 wt% or less, and Ti: 0.5 wt% Below and
Nb: A steel slab containing at least one selected from 1.0 wt% or less and adjusted so that the above C satisfies the following formula in relation to Nb and Ti at 800 ° C.
With the above, hot rolling is completed, then within 20 seconds at 20 ° C / se
Characterized by cooling at a cooling rate of c or higher, staying in a temperature range of 750 ° C or lower and 650 ° C or higher for 5 seconds or longer, then cooling at a cooling rate of 10 ° C / sec or higher, and winding at a temperature of 540 ° C or lower. The method for manufacturing a high-strength hot-rolled steel sheet according to any one of claims 1 to 4. Formula C ≧ 0.03 + Ti / 4 + Nb / 7